Factors predisposing short-tussock grasslands to Hieracium invasion in Marlborough, New Zealand

The effects of environment and management on the composition of short-tussock grasslands and the abundance of the invasive weed Hieracium pilosella were investigated in two small catchments. Species composition and site factors were recorded on a total of 182 plots and the management history of each catchment was reviewed. H. pilosella was present on >80% of all plots, but was at an early stage of invasion in one catchment (<5% cover) and dominant in the other (25% cover). Classification and ordination revealed strong between-catchment differences in community composition that reflected differences in environment (soil fertility and rainfall), disturbance history (animal populations and burning), and the stage of invasion by H. pilosella. In both catchments H. pilosella tended to be least abundant on the wettest, driest, and most fertile soils. However, such relationships were weak. Generalised additive models and regression showed that in the earlier stage of invasion individual site factors explained less than 20% of the variation in H. pilosella cover. Topographic position and slope (both indicative of soil moisture) were the most significant combined predictors, but together explained only 32% of the variation. In the later stage of invasion individual factors explained up to 33% of the variation. Topsoil sulphur, slope, and topsoil calcium were the most significant combined predictors, but together explained only 53% of the variation. Between-catchment comparisons highlighted the inter-related roles of environment, disturbance history, geographic location, availability of H. pilosella propagules, and stage of invasion in more fully explaining the abundance of H. pilosella. Of five models that have been proposed for Hieracium invasion, the: "grassland decline" model best incorporated the inter-related factors that influence spatial and temporal variation in H. pilosella abundance in the study area. This model concentrates on identifying predisposing and trigger factors that increase the likelihood of invasion and accounts for multiple causes and interactions by specifying five key factors that influence the ability of a plant species to invade existing vegetation: environment, disturbance, vegetation structure and composition, life history attributes of the invader, and the availability of invading propagules. The model potentially provides a comprehensive framework for evaluating the causes of Hieracium invasion, targeting research effort, and developing sustainable management strategies.